1. Field of the Invention
2. Description of the Related Art
This invention relates to magnesium alloys.
High pressure die cast (HPDC) components in magnesium base alloys have been successfully produced for almost 60 years, using both hot and cold chamber machines.
Compared to gravity or sand casting, HPDC is a rapid process suitable for large scale manufacture. The rapidity with which the alloy solidifies in HPDC means that the cast product has different properties relative to the same alloy when gravity cast. In particular, the grain size is normally finer, and this would generally be expected to give rise to an increase in tensile strength with a concomitant decrease in creep resistance.
Any tendency to porosity in the cast product may be alleviated by the use of a "pore free" process (PFHPDC) in which oxygen is injected into the chamber and is gettered by the casting alloy.
The relatively coarse grain size from gravity casting can be reduced by the addition of a grain refining component, for example zirconium in non-aluminium containing alloys, or carbon or carbide in aluminium containing alloys. By contrast, HPDC alloys generally do not need, and do not contain, such component.
Until the mid 1960's it would be fair to say that the only magnesium alloys used commercially for HPDC were based on the Mg--Al--Zn--Mn system, such as the alloys known as AZ91 and variants thereof. However, since the mid 1960's increasing interest has been shown in the use of magnesium base alloys for non-aerospace applications, particularly by the automotive industry, and high purity versions of known alloys, such as AZ91 and AM60, are beginning to be used in this market because of their greatly enhanced corrosion resistance.
However, both of these alloys have limited capability at elevated temperatures, and are unsuitable for applications operating much above 100.degree. C.
Some of the properties considered to be desirable in an HPDC alloy are:
a) Creep strength of the product at 175.degree. C. as good as AZ91 type alloys at 150.degree. C. PA1 b) Room temperature strength of the product similar to AZ91 type alloys. PA1 c) Good vibration damping. PA1 d) Castability of the alloy similar to, or better than AZ91 type alloys. PA1 e) Corrosion resistance of the product similar to AZ91 type alloys. PA1 f) Thermal conductivity of the product preferably better than AZ91 type alloys. PA1 g) Cost equivalent to AZ91 type alloys PA1 at least 91.9 weight percent magnesium; PA1 0.1 to 2 weight percent of zinc; PA1 2.1 to 5 weight percent of a rare earth metal component other than yttrium; PA1 0 to 1 weight percent calcium; PA1 0 to 0.1 weight percent of an oxidation inhibiting element other than calcium; PA1 no more than 0.001 weight percent strontium; PA1 no more than 0.05 weight percent silver; PA1 less than 0.1 weight percent aluminium, and PA1 substantially no undissolved iron; any balance being incidental impurities. PA1 at least 91 weight percent magnesium; PA1 0.1 to 2 weight percent of zinc; PA1 2.1 to 5 weight percent of a rare earth metal component other than yttrium; PA1 0 to 1 weight percent calcium; PA1 0 to 0.1 weight percent of an oxidation inhibiting element other than calcium; PA1 0 to 0.4 weight percent zirconium, hafnium and/or titanium; PA1 0 to 0.5 weight percent manganese; PA1 no more than 0.001 weight percent strontium; PA1 no more than 0.05 weight percent silver; and PA1 no more than 0.1 weight percent aluminium. PA1 any balance being incidental impurities.
One successful alloy development at this stage was within the Mg--Al--Si--Mn system, giving alloys such as those known as AS41, AS21 and AS11; only the first of these has been fully exploited; the other two, although offering even higher creep strengths, are generally regarded as difficult to cast, particularly since high melt temperatures are required. AS41 meets most of the objectives listed above, although its liquidus temperature is about 30.degree. C. higher than that of AZ91 type alloys.
Another series of alloys developed at about the same time included a rare earth component, a typical example being AE42, comprising of the order of 4% aluminium, 2% rare earth(s), about 0.25% manganese, and the balance magnesium with minor components/impurities. This alloy has a yield strength which is similar at room temperature to that of AS41, but which is superior at temperatures greater than about 150.degree. C. (even so, the yield strength still shows a relatively marked decrease in value with rising temperature, as will be mentioned again below). More importantly, the creep strength of AE42 exceeds even AS21 alloy at all temperatures up to at least 200.degree. C.
The present invention relates to magnesium based alloys of the Mg--RE--Zn system (RE=rare earth). Such systems are known. Thus British Patent Specification No. 1 378 281 discloses magnesium based light structural alloys which comprise neodymium, zinc, zirconium and, optionally, copper and manganese. A further necessary component in these alloys is 0.8 to 6 weight percent yttrium. Similarly SU-443096 requires the presence of at least 0.5% yttrium.
British Patent Specification No. 1 023 128 also discloses magnesium base alloys which comprise a rare earth metal and zinc. In these alloys, the zinc to rare earth metal ratio is from 1/3 to 1 where there is less than 0.6 weight percent of rare earth, and in alloys containing 0.6 to 2 weight percent rare earth metal, 0.2 to 0.5 weight percent of zinc is present.
More particularly British Patent Specification Nos 607588 and 637040 relate to systems containing up to 5% and 10% of zinc respectively. In GB 607588, it is stated that "The creep resistance . . . is not adversely affected by the presence of zinc in small or moderate amounts, not exceeding 5 per cent for example . . . ", and "The presence of zinc in amounts of up to 5 per cent has a beneficial effect on the foundry properties for these types of casting where it is desirable to avoid local4sed contraction on solidification and some dispersed unsoundness would be less objectionable". A typical known system is the alloy ZE53, containing a nominal 5 percent zinc and a nominal 3 percent rare earth component.
In these systems it is recognised that the rare earth component gives rise to a precipitate at grain boundaries, and enhances castability and creep resistance, although there may be a slight decrease in tensile strength compared to a similar alloy lacking such component. The high melting point of the precipitate assists in maintaining the properties of the casting at high temperatures.
The two British patents last mentioned above refer to sand casting, and specifically mention the desirability of the presence of zirconium in the casting alloy as a grain refining element. To be effective for such purpose, the necessary amount of zirconium is said to be between 0.1 and 0.9 weight percent (saturation level) (GB 607588) or between 0.4 and 0.9 weight percent (GB 637040).